Portable electronic devices such as cell phones and PDAs are becoming significantly smaller, thinner, and more sophisticated. Accordingly, batteries for use as the power source therefor are also required to be smaller, thinner, and higher in capacity. Promising batteries which permit small size and high capacity are lithium ion secondary batteries. Among them, flat and prismatic ones are suitable for thinning devices. Thus, such prismatic lithium ion secondary batteries are increasingly used as the power source for portable electronic devices as mentioned above.
However, lithium ion secondary batteries have high energy density and use an inflammable organic solvent as the electrolyte, so they are more likely to encounter abnormal conditions due to electrical, mechanical and thermal factors than other batteries. It is thus important to give consideration to safety. As such, lithium ion secondary batteries are usually used as battery packs, comprising a battery and a safety mechanism contained in a pack case, especially when used as the power source for portable electronic devices. More specifically, in a battery pack, a mechanism for preventing a battery from encountering an abnormal condition and, for example, a thermal fuse for preventing the battery from heating up in the event of an abnormal condition are packed in a pack case, with the thermal fuse being incorporated into the wiring. Also, in a battery pack, a lithium ion secondary battery is packed in a pack case while being connected to a circuit board with a battery protection circuit for protecting the battery from overcharge and overdischarge.
A conventional pack case is composed of a lower case for housing a battery body and an upper case serving as a lid which are joined by a bonding method such as ultrasonic bonding. However, in such a pack case, it is necessary to provide each of the lower case and the upper case with a bonding portion of certain thickness, thereby resulting in an increase in the volume and weight of the pack case.
Thus, a method as described in Patent Document 1 has been proposed. The method comprises placing a battery connected to a circuit board and the like in a mold, in this state, filling a molten resin into the mold so as to encapsulate the battery and the circuit board with the resin, and allowing the filled resin to solidify to directly form a pack case around the battery, the circuit board, and the like (see FIG. 1 of Patent Document 1). This permits a reduction of the volume of the pack case and hence the weight to a certain extent.
However, in the method of Patent Document 1, in order to heighten the capacity of a lithium ion secondary battery as a battery pack, it is necessary to form a very thin pack case to heighten the volume ratio of the secondary battery to the battery pack. However, in order to mold a very thin pack case, it is necessary to introduce expensive facilities such as the latest molding apparatus and employ a highly skilled molding technician, thereby resulting in a significant increase in costs.
To address this, Patent Documents 2 and 3 propose the following method for producing a battery pack. The method comprises: 1) placing a circuit board with a battery protection circuit, external connection terminals, etc. on the seal plate side of a secondary battery, with a space between the circuit board and the secondary battery; 2) forming a resin molded part (first molded part) in the space to produce a battery pack intermediate product comprising the circuit board and the lithium ion secondary battery which are joined integrally; and 3) covering it with a second molded part and a resin sheet to produce a battery pack (see Patent Document 2 and FIG. 2 of Patent Document 3). According to this method, a battery pack can be produced by simply wrapping a separately prepared thin resin sheet around the battery pack intermediate product including the first molded part and the second molded part which can be easily molded. Therefore, there is no need to mold the above-mentioned very thin resin around the battery, and the above-mentioned technical and cost problems are resolved. As a result, the volume ratio of the secondary battery to the battery pack can be made 85% or more without increasing the cost.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-134077    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-242947    Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-308815